1. Field of the Invention
This invention relates to the field of radio communication apparatus such as mobile telephones, portable telephones and cordless telephone. More specifically, this invention relates to a location registration of a mobile communication apparatus used as a mobile unit in a cellular radio system.
2. Description of the Related Art
Recently, many kinds of mobile communication systems have been developed and used.
One such system is a cellular radio communication system.
FIG. 5 shows a schematic view of a cellular radio communication system. This system has a control station CS, plurality of base station, for example, BS1-BS3, and plurality of mobile units, for example, MS 1-MS 7. Control star ion CS is connected to a wire telephone network NW through wire circuit CL. Base stations BS1-BS3 are connected to control station CS through wire circuits CL1-CL3, respectively. Base stations BS1-BS3 from radio zones E1-E3, which are called cells, respectively.
A group of radio communication channels are assigned to each radio zone E1-E3. Some of each of radio communication channels assigned to each base station has a different frequency. The group of radio communication channels includes a control channel and a speech channel. The control channel can be classified into two types--a paging channel (P-ch) and an access channel (A-ch). The paging channel can be used to transmit a paging signal from the base station to the mobile station. The access channel can be used to transmit an originating signal or a response signal from the mobile station to the base station. The access channel can be used to transmit a control signal which is needed for a connection between the mobile station and the base station.
In this kind of system, the base station have to know a place where the mobile station is located for paging the mobile station.
In an idle status, the mobile station searches the paging channel for example every fifteen minutes and detects the paging channel which indicates the strongest level of all the received paging channels. Next, the mobile station compares the paging channel with a paging channel which detected at the time of recent location registration. If these two paging channels are not the same channel, the mobile station recognizes that it moves to the other cell. Next, the mobile station searches an access channel and detects the access channel (Amax) which indicates the strongest level of all the received access channels. The mobile station transmits a location registration information to the base station which transmits the access channel (Amax). The location registration information includes an identification number of the mobile station. The base station registers the location where mobile station is located. The mobile station memorizes a number representing the paging channel in a memory (not shown) and goes back to the idle status. When the mobile station is located near the base station, the mobile station can detect both an access channel and a paging channel as a strongest channel from the same base station.
But when the mobile station is located on a boundary area which is formed by a plural cells, the mobile station can't detect the strongest access channel from the base station which transmits the strongest paging channel to the mobile station. The mobile station may detect the access channel and the paging channel from the different base station. In this case a paging signal can't be transmitted to the mobile station.
An explanation of this kind of error is provided next.
As shown in FIG. 6, the mobile station MS2 is located in a boundary area which is formed by cell E1 and E2. It is assumed that the location of the mobile station MS2 is registered at the base station BS1. The mobile station is going to make location registration to a base station. It is assumed that a received signal level of access channel XA1 transmitted from the base station is stronger than access channel XA2 transmitted from the base station BS2 at the input of a receiver (not shown) of the mobile station.
On the other hand it is assumed that a received signal level of paging channel XP2 transmitted from the base station BS2 is stronger than XP1 transmitted from the base station BS1. In this case mobile station MS2 makes a location registration to the base station BS1 by using the access channel XA1 and memorizes a number representing the paging channel XP2 in a memory for the location registration (not shown). So the base station (BS1) which registers the location of the mobile station MS2 does not coincides with the base station (BS2) which pages the mobile station MS2.
The mobile station MS2 moves to near the base station BS1 as shown in FIG. 7 and makes a location registration. The mobile station MS2 detects paging channel XP1 as the strongest level to the mobile station. So this paging channel XP1 doesn't coincides with the paging channel XP2 which has already memorized in the memory for the location registration (not shown).
The mobile station recognizes that it moves to the other cell and make a location registration to the base station by using the access channel XA1 which indicates the strongest level of all the access channels at the input of the receiver (not shown). The base station which registers a location of the mobile station MS2 coincides with a base station which pages the mobile station MS2. So the paging signal can be transmitted to the mobile station from the base station BS1 by using the paging channel XP1.
But, the mobile station MS2 moves to near the base station BS2 as shown in FIG. 8 and makes a location registration. The mobile station MS2 detects paging channel XP2 as the strongest level to the mobile station. So this paging channel XP2 coincides with the paging channel XP2 which has already memorized in the memory for the location registration (not shown). The mobile station MS2 doesn't recognizes that it moves to the other cell, so the mobile station MS2 recognizes that it doesn't need the location registration. The mobile station MS2 is located in cell E2, but the location of the mobile station MS2 is registered at the base station BS1. In this situation, a paging signal to the mobile station MS2 is transmitted through the paging channel XP1 from the base station BS1. So the paging signal is not transmitted to the mobile station MS2 which is located in cell E2.